1. Field of the Invention
The invention is based on a pipeline for carrying a molten salt, with a pipe wall that is stable with respect to the temperatures occurring.
2. Description of Related Art
Pipelines through which a molten salt flows are intended for use in solar power plants, for example, particularly parabolic-trough solar power plants. The pipelines are in this case connected into networks, which serve for collecting solar energy in the solar power plant. In such a solar power plant, the radiant energy of the sun is concentrated by means of parabolic mirrors onto receivers. The combination of a parabolic mirror and a receiver is known as a collector. A row of collectors is connected in series to form solar loops. The radiation energy collected by the receivers is transferred to a heat transfer fluid. At present, a biphenyl-diphenyl ether mixture is used in particular as the heat transfer fluid, which however is limited in its maximum operating temperature by its decomposition temperature of about 400° C. To obtain higher operating temperatures, making greater efficiency possible, other heat transfer fluids are required. Particularly used for this purpose are molten salts, for example that known as solar salt, a mixture of sodium nitrate and potassium nitrate in a ratio of 60:40.
However, a disadvantage of molten salts is that they have a high melting point. For example, a sodium-potassium nitrate mixture melts in the eutectic system, that is to say in a mixing ratio of 44:56, at a temperature of 218° C. In long pipeline networks, as occur in solar power plants, it is difficult to operate reliably with molten salts that have high melting points. The freezing of the molten salt in pipeline systems can cause great commercial losses. The losses are caused, for example, by the great volumetric expansion of molten salts when they melt. There is the risk of fittings and pipelines being subjected to pressure and greatly damaged.
When the molten salt freezes, which mainly takes place at times when the solar power plant is not operating, i.e. at times when the sun is not shining, there may be a volumetric contraction, which may lead to a different state of solidification, depending on the pipeline assembly and the operating state. It is likely that bubbles which are generally evacuated will occur in the pipeline and come together to form units of varying sizes. When remelting occurs, if there happens to be a great spatial distance between the locations where melting occurs with volumetric expansion and the evacuated regions, there may not be sufficient volumetric equalization to reduce the pressures occurring.
In order to prevent freezing of the molten salt, it is customary at present to drain the pipeline system during prolonged downtimes. Alternatively, it is also possible to heat the pipeline system. For this purpose, electrical energy or heat from available heat reservoirs may be used for example. If heat from available heat reservoirs is used, usually a hot heat transfer fluid is pumped through the pipeline system. These methods have the disadvantage that considerable amounts of energy in the form of electrical energy or in the form of thermal energy have to be consumed for this.
If electrical heating is provided, this is usually realized at present by laying along with the pipelines highly temperature-resistant mineral-insulated electrical heating conductors. This technique cannot be used, however, in the case of solar receivers such as are used in parabolic-trough solar power plants, since the individual receivers are thermally insulated very well from the surroundings by an evacuated glass casing. At present, receivers are therefore electrically heated by a current of high intensity being applied at a low voltage to the pipeline system itself. This has the disadvantage, however, that varying transfer resistances or thermal losses may occur at the pipeline connectors. There is an increased occurrence of electrical heat at the locations with a high resistance. Then there is the risk of heating not being uniform and the temperature locally failing to reach the melting temperature of the salt that is used as the heat transfer medium.
Internal heating conductors are known and widely used, for example in Scandinavia for the frost protection of water pipeline systems. In this case, an insulated electrical heating conductor is loosely laid in the pipeline system to be protected. When there is the risk of frost, the heating conductor prevents the pipelines from freezing. This method is thermally more efficient than heating from the outside. However, such heating conductors placed into the pipeline cannot be used for pipelines carrying molten salt. Apart from the much higher operating temperature and the oxidizing conditions of a molten salt, the internal conductor in water systems provides protection from volumetric expansion during freezing. As a difference from this, however, the volumetric expansion of molten salts does not occur during freezing but during melting.
In particular before operation commences, it is necessary to heat the pipeline system that is carrying the salt. If, for this purpose, a voltage is applied to the pipeline system itself, it is necessary before the solar power plant is put into operation to bring the entire steel mass of the pipeline system to a temperature well above the melting point of the salt. A great amount of energy is required for this purpose.
In order to handle solar power plants with long pipelines without the molten salt solidifying, it is being attempted at present to use salts that melt at a lower temperature as an alternative to solar salt. This has the disadvantage, however, that the salts have a lower thermal stability and restrict the operating range to temperatures below 500° C. This leads to lower efficiency of the solar power plant in comparison with solar salts.
It is also necessary to keep the lower-melting heat transfer salts within closed systems, which causes additional expenditure since inerting systems have to be laid in the solar array. Inerting is necessary in particular whenever nitrite-containing mixtures are used as the heat transfer salt, since, in the presence of air, the nitrite can oxidize with oxygen to form nitrate, and consequently the solidification of the salt can rise in an uncontrolled manner. If calcium-containing salt mixtures are used, the calcium may react with carbon dioxide that is contained in the air to form insoluble calcium carbonate.
Furthermore, the addition of nitrates of the elements lithium, rubidium and cesium may cause the melting point of solar salt to be lowered. However, these salts are only obtainable on a small scale and are not available cost-effectively in the amounts such as are required for solar power plants, particularly those with heat reservoirs.